In the course of assembling and operating a lithium ion cell, various process steps are usually performed to optimize performance: 1) assembly; 2) vacuum drying; 3) electrolyte and protective additives filling; 4) sealing; 5) ambient aging; 6) formation cycling; 7) elevated temperature aging; 8) degassing and re-sealing; and 9) performance checking. There may be other process steps, but these may be considered to be the relevant ones. Assembly generally includes the mounting of at least one anode, one cathode, one separator and at least two contact leads into a pouch, a can, a button cell, or other gas tight enclosure. Vacuum drying generally includes an application of vacuum and heat prior to electrolyte filling. This process may last from about 12 hours to about three days. Electrolyte filling generally can be performed by injecting an electrolyte mixture into a vacuum dried pouch assembly and then vacuum sealing the pouch. Ambient aging generally allows the vacuum sealed cell to fully adsorb electrolyte prior to cycling for the first time. Formation cycling occurs by charging the completed cell at a low rate, usually over a 12 hour or longer period, in order to form a solid electrolyte interphase (SEI) or passivation layer primarily on the anode surfaces. These layers passivate the lithium active surfaces against additional reactions. A large amount of lithium can be lost in the first formation cycle (5 to 30% of initial capacity depending on anode type), but additional losses can continue to occur. Such ongoing losses are often significant, and may be up to, or greater than an additional 20% throughout customer cycling. For some customer purposes, elevated temperature aging is usually used to pre-age the cells so that the remaining cycles are more stable from the first customer cycle to the 200th customer cycle. The elevated temperature aging step is typically performed at 50 to 60 degrees Centigrade, and may last for up to a week. During this step, additional lithium is lost along with the consumption of moisture molecules and electrolyte. The cell is then opened, degassed, and then resealed under vacuum conditions. After these steps are completed, the cell is ready for performance tests including initial capacity and capacity retention. Performance checks are made by cycling the cell at a prescribed rate and the cells are sorted for sale categories.
The losses of lithium can be categorized: (1) formation cycle building of SEI layers (primarily on the anode) by decomposition of the electrolyte; (2) the reduction of water molecules left over from the vacuum drying process and by diffusion through the package walls and seals; and (3) rebuilding of the SEI layers required due to the expansion and contraction of the active material layers (primarily in the anode). In standard (non-prelithiated) lithium ion cells, lithium is supplied by the cathode during the first charging cycle, and some cathode material forever becomes inactive as less lithium is returned to the cathode on subsequent charge cycles. This unused cathode material becomes “dead weight”. Any additional loss of lithium will further subtract directly from specific capacity. Lithium can be added to the cell prior to assembly as described by U.S. patent application Ser. No. 13/688,912, which is incorporated herein by reference, to replace first cycle losses. The amount of pre-lithiation is usually selected to avoid formation of lithium metal or dendrites on the anode; maximum anode capacity cannot be exceeded during any charge cycle, and particularly not during the initial charge cycle. There is a need to extend the cathode capacity available for cycling and maximize the specific capacity for the target number of customer cycles.
The depletion of electrolyte occurs during: 1) The formation cycle and formation of initial SEI layers, primarily on the anode; 2) The elevated temperature aging cycle where additional electrolyte is consumed; and 3) Customer cycling. Reducing these losses could increase cell lifetime. Reducing electrolyte consumption could stabilize cell resistance and improve capacity retention. Reducing the consumption of electrolyte additives could reduce cell cost.